Intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for assessing a vessel, such as an artery, within the human body to determine the need for treatment, to guide intervention, and/or to assess its effectiveness. An IVUS imaging system uses ultrasound echoes to form a cross-sectional image of the vessel of interest. Typically, IVUS imaging uses a transducer on an IVUS catheter that both emits ultrasound signals (waves) and receives the reflected ultrasound signals. The emitted ultrasound signals (often referred to as ultrasound pulses) pass easily through most tissues and blood, but they are partially reflected at impedance discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. The IVUS imaging system, which is connected to the IVUS catheter by way of a patient interface module, processes the received ultrasound signals (often referred to as ultrasound echoes) to produce a cross-sectional image of the vessel where the IVUS catheter is located.
One preferred type of ultrasound transducer for IVUS imaging is the piezoelectric micromachined ultrasound transducer (PMUT), which is a microelectromechanical system (MEMS) device, typically fabricated in large batches on a silicon wafer substrate. MEMS fabrication techniques are used to produce thousands of PMUTs on a single silicon wafer. Typically, a PMUT may be formed by depositing a piezoelectric polymer onto a micro-machined silicon substrate. The silicon substrate may also include electronic circuitry used to provide an electrical interface to the transducer. Alternatively, the electronic circuitry associated with the PMUT may be contained in a separate application-specific integrated circuit (ASIC) which is located in close proximity to the PMUT device and connected by electrical leads. The PMUT MEMS device with its associated electronic circuit (either included on the same substrate or located on a separate adjacent ASIC), with an attached length of electrical cable is referred to as a tadpole assembly based on its configuration consisting of a somewhat bulbous transducer assembly coupled to a long tail-like electrical cable. Currently, PMUT tadpole assemblies are coated with Parylene to insulate the front electrode and other electrical connections from contact with fluids (e.g., saline or blood). This is inconvenient, since it is complicated to introduce a large number of tadpole assemblies into the Parylene chamber and to protect the attached electrical cables from being coated.
Therefore, while conventional wafer fabrication techniques and methods of coating a protective layer on transducer assemblies at the tadpole stage are generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.